This invention relates to insulated tubing for conveying vapors such as the injection of steam downwardly into an oil-bearing formation to enhance oil recovery, the tubing being of the type having plural segments of insulated, double-walled tubing coupled to form a long string and, more particulary, to such a tubular apparatus which is provided with a cylindrical insert at one or more of the joints of the tube segments to reduce heat loss at the joints.
The injection of pressurized, high temparature steam into oil-bearing formations has become a significant method of enhancing oil recovery. In a typical arrangement, segments of insulated, double-walled tubing are connected together by threaded sleeve couplings. Each segment of the double-walled tubing structure is composed of inner and outer concentric tubes radially separated by an annulus which is sealed at each end of the tubes and provided with an insulating medium. Adequate insulation of the injection string is desired to assure a maximum steam quality at the bottom of the string.
The annulus is sealed at the ends of each tubing segment by seal rings or bushins welded to the inner and outer tubes, or by outwardly flaring the ends of the inner tube and welding the inner and outer tubes to each other at their ends. The outer surface of each of the extremities of the two outer tubes of each pair of adjacent segments are received in the bore of a coupling wherein they are connected by a threaded engagement. Since the inner and outer tubes are connected together in the coupling area, a significant quantity of heat can be lost through the weld or other connection means between the inner and outer tubes and through the coupling.